1. Field of the Invention
The present invention relates to a voltage booster circuit. More particularly, the present invention relates to a voltage booster circuit increasing an input voltage by using charge/discharge of a capacitor to generate a desired boost voltage and a semiconductor device for incorporating the voltage booster circuit.
2. Description of the Related Art
To accomplish a lower voltage of a voltage source or a lower current consumption, various semiconductor devices in which a voltage booster circuit is incorporated is becoming increased. The voltage booster circuit boosts a source voltage or an internal source voltage which is generated from the source voltage and lower than the source voltage, and generates the source voltage or a boosted voltage higher than the internal source voltage.
In particular, it is required to provide a voltage booster circuit as a power supply to liquid crystals in the semiconductor device for a liquid crystal display using various kinds of voltages.
As a conventional voltage booster circuit, for example, there is a circuit to boost a source voltage or an internal source voltage by utilizing the charge/discharge of a capacitor.
The voltage booster circuit as mentioned above connects, for example, a plurality of N channel MOS transistors in series between an input terminal and an output terminal of the booster circuit and a capacitor is connected to a connecting point of each of the N channel MOS transistors. According to a voltage level of a signal 0, the conductive state in N channel transistors of even number and odd number connected in series is alternately controlled, and at the same time the charge and discharge of the capacitor is alternately controlled. By repeating this cycle, the voltage is boosted stepwise and thereby a desired boosted voltage is obtained as an output voltage.
However, when the voltage booster circuit is constituted as mentioned above, there is a possibility that an initial status of the voltage booster circuit is not stable at an initial operation stage, i.e., after a power supply is turned on. This is because that since the boosted voltage generated by the voltage booster circuit is used as a voltage which is supplied to each of the transistors which constitute the voltage booster circuit it may happen sometimes that this boosted voltage is not supplied to each transistor and thereby, the voltage booster circuit is not activated. This is because the N channel MOS transistors are connected in series and any of the N channel MOS transistors is non conductive state right after the power is turned on. Therefore, this voltage may not be transmitted to an output terminal even if an input voltage is applied to an input terminal.
As a solution of such a drawback, a transistor of depression type is used for the N channel MOS transistors. However, a step for producing the depression typed transistor needs to be added in the process of manufacturing the semiconductor device and, therefore, the cost for manufacturing the semiconductor device is increased. Further, the transistor of the depression type is always conductive state, and loss for a boosted voltage level occurs or the current consumption is increased.
In addition, in recent years, it is required that users can set a voltage to the desired boosted voltage. Though the voltage can be set to a desired value by coupling a capacitor to an outside of the circuit, care for a wire is becoming necessary so that the capacitor can easily be coupled.